1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory, particularly to an OTP (One-Time Programmable) memory using an anti-fuse element.
2. Description of the Related Art
Recently, a nonvolatile memory which continuously retains information under the power off has been widely spread. In some applications of the nonvolatile memory, data is not repeatedly rewritten, but the data is written only once. As one of such applications, a nonvolatile semiconductor memory using an anti-fuse element has been proposed.
The anti-fuse element initially becomes a high-resistance state by a function of an insulating film in the anti-fuse element. However, when a high voltage for an electric stress is applied to the anti-fuse element once, a composition of the insulating film is broken to lower an electric resistance. Thereby, one-bit data can be recorded in a nonvolatile manner. In order to prevent false read, it is necessary to completely break the insulating film.
Generally, the breakdown feature of the insulating film depends on an applied voltage and an environmental temperature such that the time the insulating film is completely broken extends as the applied voltage or the environmental temperature decreases. Therefore, depending on the applied voltage or the environmental temperature, the insulating film is sometimes incompletely broken when the program operation is shortened. On the other hand, when the program operation is excessively lengthened, the excessive electric stress applied to the anti-fuse element, which causes a problem from the viewpoint of reliability.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2006-196079 discloses a technique in order to solve the problems. In the technique disclosed in JP-A No. 2006-196079, a function of sensing an insulation breakdown state is provided in a power supply circuit that supplies a voltage to the anti-fuse memory element, and the application of the voltage to the anti-fuse memory element is maintained until the insulating film is completely broken. Therefore, not only a good read characteristic is obtained, but also the excessive electric stress is not applied to the anti-fuse memory element because the program operation is performed only for a necessary time.
However, in such cases, there is a problem that a program time varies by the applied voltage or the environmental temperature.